But going underground here on Earth also insulates you from like 99% of planet-scale disaster scenarios, especially if you're growing your crops underground and recirculating your breathing air, both of which you'd be doing in a Mars underground settlement.

I'm totally down for how nifty it would be to have off-world colonies, but trying to sell it to the public with these "what if" arguments doesn't even begin to pass the sniff test. Gates has been sounding the alarm for years about viral pandemics and basically nothing was prepared for it; how capable would we be of spending trillions on a "backup Earth" when we can't even spend a few million a year on stockpiling medical PPE?

fully agree - but there is a middle ground. Antarctica has maintained permanent year round settlements for various purposes over the last century. These settlements exist for no other reason than there are both scientific and economic resources unique to Antarctica.

Barring a general change in the habitability of the Moon/Mars or other astronomical body I'd wager that you would have a similar dynamic. If unique economic purposes are discovered including rich surface deposits, low-G fuel generation, or lower environmental concerns then the population in these settlements would grow for the same reasons that people have always moved to the frontier.

Which economic resources? I always thought research settlements are a money sink. Or are you speaking of the little bit of tourism?

Research settlements are funky, funding agencies and scientists would be quite interested in understanding the geology dynamics on a tectonically dead world. Biologists will want to understand why mars never developed life. Medical researchers will want to understand the effects of isolation, and low-g. Donors, and funding agencies interested in these fields will want to fund expeditions. The expeditions will need infrastructure. The infrastructure's existence will motivate a few entrepreneurial expeditions.

Once the transit economics are understood, The Moon and Mars can both provide accessible surface deposits of water, iron, copper, and other commodities to orbit for less energy than earth. A number of activities for resource procurement for commercial space industries will be a tad easier when attached to a big low-g rock where an extra green house can simply be dug out. The relative lack of environmental externalities from mining and other activities may be a resource unto itself in the next few decades.

On a longer timeline - Mars could be given a tolerable atmospheric pressure and liquid water for <100 trillion dollars. Barring a change in global growth rates that would be ~3% of annual global GDP by 2120, and .1% by 2220.

Err, thanks. I meant the ones in Antarctica, or did I get that wrong?

ahh no - mistake was mine. Antarctica has been home to whaling colonies in previous time periods.

Yes, I know that. But we don't do that anymore, execpt the Japanese.